As processes for producing acrylic acid, from old times there have been known a process in which acetylene, carbon monoxide and water are made to react together in the presence of a nickel catalyst (so-called Reppe process) and a process in which acrylonitrile is hydrolyzed. However, thereafter, a process of two-step gas phase oxidation of propylene, namely, a process in which propylene is air-oxidized to convert it into acrolein and then this acrolein is further oxidized to convert it into acrylic acid, has been developed and now is industrially commonly employed (see, for example, JP-A-01-063543 (Kokai) and JP-A-55-102536 (Kokai)).